As someone who has caught student plagiarism, I can tell you--it is actually (at my R1 university, anyway) very rare for a student to get kicked out for a single incidence of plagiarism. It is most likely, as others have mentioned, that you receive a zero on that assignment (and in some cases, you might just receive a written warning). We keep our papers from our students for one semester after the course is completed. After that time, we don't keep track of it anymore.
I can say, a number of my undergraduate papers were bad. Like really bad. Not plagiarized, but I would be embarrassed to have those horrible papers be reviewed by anyone. The truth is, unless you submit the paper as a writing sample or otherwise publish it, after you turn it in, it is long forgotten by the faculty and university. Unless you've given someone a reason to, no one is combing through your undergraduate essays to see if you paraphrased poorly on one essay.
I would not, in the slightest, worry about 8 poorly paraphrased words. I would caution you, however, that if you are getting a PhD (or already have one), you need to be careful to not do this again in the future. More than your degrees, you risk your credibility.